1、Designation: D4517 04 (Reapproved 2009)Standard Test Method forLow-Level Total Silica in High-Purity Water by FlamelessAtomic Absorption Spectroscopy1This standard is issued under the fixed designation D4517; the number immediately following the designation indicates the year oforiginal adoption or,
2、 in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of total silicain water.1.2 This test
3、method is applicable in the range from 25 to250 g/L of silica as SiO2. Higher concentrations may bedetermined by decreasing the aliquot volume (see Note 6).Concentration range should not be extended by dilution.1.3 This test method determines total silica, and does notdistinguish between soluble and
4、 insoluble forms.1.4 This test method was tested on reagent water only. It isthe users responsibility to assure the validity of the testmethod for waters of other matrices.1.5 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof th
5、e user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D859 Test Method for Silica in WaterD1066 Practice for Sampling SteamD1129 Terminology Relating to WaterD1192 G
6、uide for Equipment for Sampling Water and Steamin Closed Conduits3D1193 Specification for Reagent WaterD2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD3370 Practices for Sampling Water from Closed ConduitsD3919 Practice for Measuring Trace E
7、lements in Water byGraphite Furnace Atomic Absorption SpectrophotometryD4453 Practice for Handling of Ultra-Pure Water SamplesD5810 Guide for Spiking into Aqueous SamplesD5847 Practice for Writing Quality Control Specificationsfor Standard Test Methods for Water Analysis3. Terminology3.1 Definitions
8、: For definitions of terms used in this testmethod, refer to Terminology D1129.4. Summary of Test Method4.1 Total silica is determined using an atomic absorptionspectrophotometer in conjunction with a graphite furnace. Asample is placed in a graphite tube, evaporated to dryness,charred, and atomized
9、. Since the graphite furnace uses thesample much more efficiently than flame atomization, thedetection of low concentrations of elements in small samplevolumes is possible. Finally, the absorption signal duringatomization is recorded and compared to standards. A generalguide for the application of t
10、he graphite furnace is given inPractice D3919. Pretreatment of the graphite tube may be usedto enhance the sensitivity and repeatability, or both, of the test.44.2 Total silica is determined using a freshly ultrasonicallytreated and shaken aliquot of sample.4.3 This test method determines low-level
11、total silica inhigh purity water. Refer to Test Method D859, Method B, fordetermination of molybdate-reactive silica.5. Significance and Use5.1 Control of silica in boiler feedwater and boiler water isnecessary to minimize the formation of scale-forming silicatesthat decrease heat transfer in the bo
12、iler. Volatilization andcarryover of silica with the steam may cause hard, glassysiliceous deposits to form on turbine blades that reduce turbineefficiency.5.2 Colloidal silica that is not removed by boiler waterpretreatment processes may be solubilized in the boiler and1This test method is under th
13、e jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling Water andWater-Formed Deposits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved Oct. 1, 2009. Published N
14、ovember 2009. Originallyapproved in 1985. Last previous edition approved in 2004 as D4517 04. DOI:10.1520/D4517-04R09.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer t
15、o the standards Document Summary page onthe ASTM website.3Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.4Rawa, Judith A. and Earl L. Henn, “Determination of Trace Silica in IndustrialProcess Waters by Flameless Atomic Absorption Spectrometry,” Analytic
16、al Chem-istry, Vol 51, March 1979.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.thus contribute to the dissolved silica concentration in theboiler. Both dissolved and total silica are of interest.6. Interferences6.1 For a complete
17、discussion of general interferences withfurnace procedures, the analyst is referred to Practice D3919.6.2 Relatively pure waters such as demineralizer effluent,condensate, or high-pressure boiler feedwater are of sufficientpurity to minimize potential interferences such as sodium,orthophosphate, or
18、sulfate.6.3 Graphite tube pretreatment with the calcium/lanthanumreagent has been found to improve the silica response withsome graphite tube materials or designs. Since responseproblems may include memory effects as well as poor sensi-tivity, the following procedure must be used to determinewhether
19、 pretreatment is needed.6.3.1 Following instrument set-up and blank determinationas described in 10.1-10.3, inject 10 to 12 replicates of astandard containing 100 to 150 g/L of SiO2. Treat the tube inaccordance with 6.4, and repeat. Note whether the response ofthe two sets is constant, indicating no
20、 memory effect, orwhether the first set shows increasing response. Note alsowhether the response of the second set is greater than that ofthe first. If either sensitivity of repeatability is improved, tubepretreatment is recommended.6.4 Pretreatment may be accomplished as follows:6.4.1 Inject into t
21、he furnace the diluted calcium/lanthanumreagent using the maximum aliquot recommended by themanufacturer of the tube, and start the atomization program.Repeat three times, or more if necessary. Run blank atomiza-tion cycles until the furnace blank is constant.6.4.2 Proceed to calibrate the furnace a
22、nd analyze samplesin accordance with Sections 10 and 11. The absorbanceenhancement may deteriorate after numerous injections, inwhich case retreatment will be required. Refer to PracticeD3919.7. Apparatus7.1 Atomic Absorption Spectrophotometer, for use at 251.6nm.NOTE 1A wavelength other than 251.6
23、nm may be used if it has beendetermined to be equally suitable.NOTE 2The manufacturers instructions should be followed for allinstrument parameters.7.2 Silicon Light Source, silicon hollow cathode lamp.7.3 Graphite Furnace, capable of reaching temperaturessufficient to atomize the element of interes
24、t. Atomizationtemperature must be reached as rapidly as possible for maxi-mum sensitivity.NOTE 3Special furnace accessories are commercially available toprovide a means of accelerating atomization temperature.7.4 Graphite Tubes, compatible with furnace device. Stan-dard graphite tubes are preferred,
25、 particularly when tubepretreatment is practiced. Pyrolytic graphite tubes may be usedif it has been determined by the analyst that the precision, bias,and sensitivity of the determination are not compromised.7.5 Pipets:7.5.1 Microlitre, with disposable, polypropylene tips. Sizesmay range from 1 to
26、1000 L, as required.7.5.2 Millilitre, 50.0 mL.7.6 Data SystemFast transient signal data are collectedand processed using an internal microprocessor or externaldesktop computer systems. Data can be stored on disks,transmitted to central servers, or printed in hard copy. Datamay be evaluated and proce
27、ssed using the instruments dedi-cated systems to determine analyte concentrations. Users ofthis practice may use a strip chart recorder to obtain sample andcalibration data, if desired.7.7 Ultrasonic Cleaner, operable at 20 KHz.7.8 Flasks, volumetric, 100 and 1000-mL polyethylene.8. Reagents and Mat
28、erials8.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Societywhere such specifications are available.5Other grade
29、s may beused, provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the determination.8.2 Purity of Water All references to reagent water in thismethod shall be understood to conform to Specification D1193,for reagent water
30、 Type II. In addition, the reagent water shallbe made silica-free and determined as such in accordance withthis method. The collecting apparatus and storage containersfor the reagent water must be made of suitable materials thatwill not contaminate the reagent water with silica (see Section9).NOTE 4
31、Silica-free water may be prepared by distillation, demineral-ization, and passage through a 0.22 m filter. Total removal of colloidalsilica may be done by treating the above water with hydrofluoric acid,letting it stand for 24 h, and then processing it through strong baseion-exchange resin in the hy
32、droxide form.8.3 Calcium/Lanthanum ReagentWet 2.90 g of lanthanumoxide (La2O399.99 % pure) with about 50 mL of water in a100 mL volumetric flask. Carefully add 5.5 mL of concentratednitric acid (HNO3sp gr 1.42) and swirl to dissolve. Add 0.100g of calcium oxide (CaO) and swirl to dissolve. Dilute to
33、volume with water. This solution contains 100 mg/L CaO and2500 mg/L La. Dilute 1:100 for injection. The diluted reagentis needed only for pretreatment of the graphite tube asdescribed in 6.4.8.4 Silica Stock Solution (1 mL = 1 mg SiO2) Dissolve 4.7g of sodium metasilicate (Na2SiO39H2O) and dilute to
34、 1 Lwith water. Determine the silica concentration of this solutionin accordance with Test Method D859, Method A.8.5 Silica Working Solution (1 mL = 0.05 mg SiO2)Dilute50.0 mL of the silica stock solution (see 8.4) to 1 L with water5Reagent Chemicals, American Chemical Society Specifications , Ameri
35、canChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmaceutical Convention, Inc. (USP
36、C), Rockville,MD.D4517 04 (2009)2in a volumetric flask. The concentration of this diluted standardis calculated after confirming the concentration of the stocksolution.8.6 Argon, standard, welders grade, commercially avail-able.9. Sampling9.1 Collect the sample in accordance with the applicableASTM
37、standards: Practice D1066, Specification D1192, andPractices D3370.9.2 Since silica is such a ubiquitous material, meticuloussample collection, handling, and injection into the furnace arenecessary to avoid contamination.9.2.1 Collect samples only in plastic or TFE-fluorocarboncontainers. Polystyren
38、e, polypropylene, linear polyethylene,and TFE-fluorocarbon have been found to be generally suit-able, but tests should be conducted on sample containers beforeuse to determine their suitability.9.2.2 Sample containers must be ultrasonically cleanedbefore use. Fill the sample containers with water an
39、d immersein an operating ultrasonic cleaner for at least 1 min. Remove,shake vigorously, and drain. Additional cycles of cleaning maybe found necessary.9.2.3 Before actual sample collection, rinse sample con-tainer with the sample it is to contain three times with vigorousshaking, then fill. Do not
40、allow any object to touch the insideof the bottle or cap.10. Calibration10.1 Prepare standards for test calibration containing about0, 50, 100, 150, and 250 g/L of SiO2by diluting 0, 100, 200,300, and 500 L of the silica working solution (see 8.4)to100mL with water in separate volumetric flasks.NOTE
41、 5The exact concentration of the prepared silica standards isdetermined from the result obtained by the gravimetric standardization ofthe stock solution in 8.4.10.2 Zero the instrument without making an injection orinitiating an atomization program.10.3 Set the atomization program (drying, charring,
42、 atom-izing) according to the manufacturers instructions.NOTE 6While sample size and furnace programming vary amonginstruments, the following analytical conditions have generally beenfound to be suitable. It is up to the analyst to optimize analyticalconditions for the instrument used.AliquotAn aliq
43、uot size of 10 to 100 L, depending on tube size andconcentration of sample. The same aliquot size must be used for allstandard, sample, and blank injections.Function Temperature, C Time, sDry 100 1 s/LChar 1000 30Atomize 2800 710.4 Determine the furnace blank by initiating the atomiza-tion program w
44、ithout making an injection. Repeat until thefurnace blank reproduces within 10 %.NOTE 7Determination of the furnace blank is a mechanism forevaluating the acceptability of the graphite tube for silica determination.10.5 Inject a measured aliquot of the 100 g/L of SiO2standard solution (see 10.1) int
45、o the furnace, and initiate thepresent atomization program. Repeat this analysis until theresponse reproduces within 10 %.10.6 Determine the acceptability of the water by injecting ameasured aliquot of the “0” standard solution into the furnaceand initiating the preset atomization program. Repeat th
46、isanalysis until the response reproduces within 10 %.10.7 Inject a measured aliquot of each of the remainingstandard solutions as described in 10.5.10.8 Prepare a calibration curve by plotting absorbance orpeak height versus concentration as micrograms per litre ofSiO2on linear paper if direct reado
47、ut in terms of concentrationis not possible with the instrument.11. Procedure11.1 Rinse the microlitre pipet with water and inject analiquot of water to test pipet and furnace cleanliness beforesample analysis. After samples containing a high concentrationof silica are analyzed, additional atomizati
48、on cycles may beneeded to reduce the furnace blank to prior level.11.2 Ultrasonicate for at least 1 min and then vigorouslyshake sample. Immediately withdraw an aliquot (refer to 10.3and Note 6) with the microlitre pipet. Do not touch pipet tip toany object.11.3 Analyze sample as described in 10.2-1
49、0.4.NOTE 8If the total silica concentration of the sample is below thedetection limit with the normal maximum injection volume for theinstrument used, multiple injections may be used as follows. Inject analiquot, start program. Stop at completion of drying step. Repeat asrequired. Allow program to run to completion when enough sample hasbeen injected. Both precision and bias will decrease when this techniqueis used.12. Calculation12.1 Sample concentration is read directly from calibrationcurve prepared in 10.8. If multiple injections are used asdiscuss
